pyqt5 2nd window not loading correctly

Question

When I try to load my 2nd window it is giving me a small blank window without any of my setup from the UI file. It is closing the Main welcome window just fine, but the window with all the forms and setup it isn't loading.

class AppManager(QMainWindow):

    def __init__(self):
        super(AppManager, self).__init__()

        self.Main = Main()
        self.FN5 = FN5()

        self.Main.start.clicked.connect(self.chooseLine)

        self.Main.show()

    def chooseLine(self):

        if self.Main.sd1.count(self.Main.line_combobox.currentText()) and self.Main.shift1.isChecked():
            self.FN5.show()
            self.Main.close()

1st window

class Main(QMainWindow):

    def __init__(self):
        super(Main, self).__init__()

        self.sd1 = ["FN05"]
        self.sd2 = ["FN01", "FN02", "FN03", "FN04"]
        self.ps1 = ["FN06", "FN07", "FN08", "FN09", "FN10"]
        self.fn_Line = []

        for x in self.sd1:
            self.fn_Line.append(x)
        for x in self.sd2:
            self.fn_Line.append(x)
        for x in self.ps1:
            self.fn_Line.append(x)

        self.fn_Line.sort()

        uic.loadUi("finishing.ui", self)

        self.line_combobox = self.findChild(QComboBox, "FN_Line_comboBox")
        self.start = self.findChild(QPushButton, "pushButton_begin")
        self.shift1 = self.findChild(QRadioButton, "radioButton_1")

        self.line_combobox.addItems(self.fn_Line)

        self.fn_Line_comboBox.setCurrentIndex(-1)

2nd window

 class FN5(QMainWindow):

    def __init__(self):
        super(FN5, self).__init__()

        self.window = QMainWindow()
        self.ui = Ui_Fin5()
        self.ui.setupUi(self.window)